POST-ENDODONTIC RESTORATION ************************************************************* INTRODUCTION WHY DO WE NEED TO RESTORE ENDODONTICALLY TREATED TEETH? FUNCTIONS WHEN DO YOU PROVIDE A POST-ENDO RESTORATION? BASIC COMPONENTS OF A POST-ENDO RESTORATION THE POST THE CORE THE FINAL RESTORATION BIOMECHANICAL PRINCIPLES OF TOOTH PREPARATION CLINICAL TECHNIQUES CONCLUSION
INTRODUCTION Root canal therapy cannot be summarized by saying, “fill it, shut it, forget it”. The final restoration over an endodontically treated tooth is as important or probably even more important than the actual root canal therapy itself. The endodontically treated tooth constitutes a unique subset of teeth requiring restorations because: First and foremost, the tooth structure has already been weakened by previous episodes of caries, trauma, restorative procedures and tooth preparations. An accountable percentage of structural integrity is lost because of the root canal therapy itself due to access cavity preparation and canal instrumentation leading to increased flexing of the tooth and ultimately its fracture. Some authors quote an alteration in the composition of the dentin such as a change in the collagen cross linking, dehydration etc. Of course, whether such a change is actually detrimental to the prognosis of the treatment is questionable as studies have not been very conclusive about it. Moreover such teeth are esthetically and physically compromised requiring special kinds of restoration. Finally and most importantly, the RC treated tooth has got an impaired neurosensory feed- back mechanism because of the lack of pulpal tissue i.e. the protective property of ‘proprioception’ is lost. This renders the tooth more vulnerable to fracture under normal masticatory forces. So, a person can unintentionally bite too hard on the RC treated tooth compared to a vital tooth, which can lead to its fracture and failure. All these points emphasis the need of a wisely designed, accurate and strong restoration over the endodontically treated tooth, which would not only protect the tooth
but also prevent any form of microleakage. Coronal leakage or orthograde contamination via saliva has now been sited as the prime cause of endodontic failures. Hence the functions of a post-endodontic restoration are: -
Providing a coronal seal.
Providing protection to the remaining tooth structure.
Restoring the esthetics and functions of the tooth. Treatment planning for the post-endodontic restoration should start, ideally
speaking, prior to the commencement of the RCT itself. An extraction should be carried out if the restorability and periodontal status of the tooth is questionable instead of spending unnecessary time and expenses on it. If this is not the case and the tooth is salvageable, then the following principles should be kept always in mind: 1. Preservation of remaining tooth tissue by conservative restorative design compatible with acceptable aesthetics and function. 2. Reduction of stress and its favorable distribution within the remaining tooth tissue by proper design and selection of materials. Now, an important question before us is when to restore after endodontic treatment. Most authors agree that since the success rate of RCT is pretty high at 80-95%, an arbitary period of a couple of weeks is sufficient before providing a permanent restoration. A permanent restoration may be delayed if the endodontic treatment has: -
Doubtful prognosis requiring possible re-treatment.
Presence of sinus
Tenderness to palpation /percussion
Basic components of the post-endodontic restoration are: -
The final restoration
Simply put, the post helps in the retention of the core, the core helps in the replacement of the missing tooth structure and the final restoration helps in restoring the form and function of the tooth and protects the remaining tooth structure from fracture. Not all teeth require a post or a crown. Some require all 3 components while some may need just an access seal.
THE POST Definition: The post can be defined as a relatively rigid restorative material placed in the canal of a non-vital tooth. Function: The post has got 2 main functions: -
Retention of core
Protection of the tooth by dissipation of forces along its long axis to the surrounding periodontal tissues and the alveolar bone.
Ideal properties: -
Retentive to the tooth and the core
Strong and provide protection to the tooth
Esthetic wherever required
Extensive coronal tooth structure loss/ >50% loss of tooth structure including important landmarks such as marginal ridges, cingulum, reciprocating walls etc.
Teeth which are going to be used as abutments for FPDâ€™s, overdentures etc
Teeth exhibiting extensive wear or patients having any parafunctional habits which can cause excessive amounts of load on the tooth.
Teeth that are going to become a part of a periodontal splint.
For the retention of a complex restoration.
Contraindications: Besides the opposites of the above points, certain other contraindication include: -
Endodontically treated teeth having a questionable prognosis requiring possible re-treatment.
Teeth having minimal canal dentin
Teeth having unusual anatomy such as extreme curvatures and fragile roots.
Classification: Numerous classifications have been quoted by several authors, which are more or less similar to each other. The following is a simple way of classifying posts based on their features: 1. Fabrication: Custom cast Prefabricated 2. Materials used: Metallic Non-metallic 3. Esthetics: Esthetic Non-esthetic 4. Retention mode: Active / threaded Passive/ cemented 5. Shape: Parallel Tapered Parallel with apical taper 6. Surface configuration: Smooth Serrated Threaded 7.Shank configuration:
Solid Hollow Split
9. Resiliency: Rigid Flexible Features: The posts, as mentioned in the classification, come in various diameters, lengths, tapers, sizes, surface configurations, shank configurations etc. The choice of an individual post selection is purely based on the operatorâ€™s discretion and the type of case before him.
Types of posts: Posts can broadly be of 2 types â€“ custom cast or prefabricated. Custom cast posts are those posts which are made by taking an impression of the canal space either directly or indirectly and casting a post which exactly replicates the canal space. The prefabricated post is available commercially in predetermined dimensions, length and contours and hence does not replicate the canal space. Some major differences between the 2 are:
The prefabricated metal post can be of 2 types based on their mode of retention - Passive post or the cemented post which rely on a cementing media for their retention
- Active post or the threaded post which actively thread into the dentin to attain retention.
The non-metallic prefabricated posts can be either esthetic posts or non-esthetic posts.
THE CORE Definition: The core can be defined as a restorative material placed in the coronal portion of the tooth to replace the missing coronal tooth structure.
The core can be anchored to the tooth either via its extension onto the coronal aspect of the tooth or via an endodontic post. Ideal properties:High compressive strength -
Should not deform plastically under loading
Retentive to post, tooth and if possible to the overlying crown as well
Should be easy to manipulate and place
Should set rapidly
Esthetic wherever indicated
Core materials: Cast core
Advantages High strength
No concern regarding
delamination from the post High compressive strength
Casting inaccuracies Unesthetic
High modulus of elasticity
Cannot be bonded to
Easy manipulation and
Tendency to discolour
Stable to thermal and
Tendency to corrode
Low early strength â€“ preparations cannot be done
immediately Microleakage due to
Command set- preparation
Tendency to deform
can be done immediately
plastically and thus cannot
Chemically adhesive to the
be used in high stress areas Low fracture resistance
Early moisture sensitivity
Used when>50% of tooth RMGIC
structure is intact Properties lie in between
Tendency to expand in
composites and GIC
presence of moisture- can
lead to fracture of overlying
More adhesive than GIC
Decreased moisture sensitivity Decreased microleakage
Pin-retained cores: At times posts cannot be placed in the root due to anatomical factors or due to the non-irretrievability of a silver cone obturation. In such situations, pins can be used to gain an adequate amount of retention. Sometimes pins can also be used along with a post to gain an auxiliary mode of retention. Amalcore: Nayyar and Walton have described a technique wherein the coronal 2mm of guttapercha is removed and amalgam condensed into it to attain adequate amount of retention for the core. Studies have shown favorable results with this type of technique.
FINAL RESTORATION: The final component of a post-endodontic restoration is the final or the definitive restoration. As mentioned before, the final restoration helps to Protect the remaining tooth structure from fracture Protect the entire unit from microleakage Restore the esthetics and functions of the tooth Posterior teeth:Because of the vertical forces experienced by a posterior tooth, as a general rule all endodontically treated posterior teeth require a full occlusal coverage restoration. This could be a metal / a ceramo-metal crown or it could be an onlay. Anterior tooth:Anterior teeth usually experience shear forces. Depending on the structural integrity and the amount of discolouration of the tooth of the tooth, a permanent restoration for an anterior tooth can take any of the following forms:
Teeth with posts = Jacket crowns-all-ceramic/ ceramo-metal
Teeth without posts but discolored = Laminate veneer /JC
Intact teeth with no discoloration = Composite for the access seal PREPARATION PROCEDURES
Biomechanical principles governing placement of post-endodontic restorations: ď ś CONSERVATION OF TOOTH STRUCTURE Canal: Care should be taken to remove minimum amount of tooth structure from within the canal space to prevent the fracture of the root either during cementation or function. If the cross section of the tooth is considered as a ring, then the strength of the ring is directly proportional to R4 â€“ r4 where R= radius of the tooth and r= radius of canal i.e the strength is drastically effected by a substantially sized post. Thus, the canal should be enlarged just enough so as to seat a post passively yet accurately at the same time ensuring retention and rigidity for the post. Ideally, the post diameter= 1/3 the diameter of the tooth or the post should have a minimum of 1mm of sound dentin surrounding it. Coronal tooth structure: As much of the coronal tooth structure should be preserved to avoid any kind of stress build-up especially at the narrowest circumference of the tooth which is the cervical 1/3 of the tooth. This prevents any chance of a fracture, which can occur during function. Time and again, it is emphasized that a minimum of 2mm of tooth structure is compulsory for the success of a post endodontic restoration. Why is this so? It is because; this remaining vertical height of tooth is necessary to accommodate the ferrule or the extra-coronal brace, which is going to be provided by the subsequent full coverage metal crown/coping. What is a ferrule? A ferrule is defined as a 3600 metal collar of the crown or coping surrounding the parallel walls of the dentin extending coronal to the cervical line of the preparation resulting in an elevation of resistance form for the tooth. Stresses in radicular dentin get concentrated at the narrowest circumference of the tooth. If no ferrule is provided then constant forces on the tooth can lead to a vertical splitting of
the root during function. Providing an encasement of binding of the gingival 2mm of the axial walls of the preparation reduces the incidence of fracture of the tooth by reinforcement and dissipation of forces that concentrates at the narrowest circumference of the tooth. Thus, a ferrule: -
Helps to bind the remaining tooth structure together
Prevents root fracture during function
Prevents stress concentration at gingival margin
To be successful, the ferrule must encircle a minimum of 2mm of vertical wall of sound tooth structure between the cervical finish line and gingival extent of the core. In case the remaining tooth structure is inadequate, then periodontal crown lengthening surgery or orthodontic extrusion is necessary. This effect can easily be understood by a simple example : When a nail is hammered into a wooden peg, the latter tends to split along its long axis. If a metal ring is placed to brace the impact, no such splitting occurs. According to Wein and Ingle, if adequate tooth structure is present, a 45 0 bevel can be placed in the occlusal aspect of the preparation to allow for an extension of the core around the axial walls. This extra metal collar of the core gives extra resistance to fracture and also acts as a stop against over seating / wedging action of the post and core. If inadequate, crown lengthening surgery or orthodontic extrusion should be done.
ď ś RETENTION FORM:
Post retention can be defined as the ability of a post to resist vertical dislodging forces. This is affected by: -
Preparation design For circular canals
For elliptical canals
restricted taper of 60-80
Post design: Active > passive parallel> passive tapered
Post length: Îœore the length, more the retention Ideally, length of post = Crown Length or at least 2/3-3/4 the length
Post diameter: Îœore the diameter more is the retention but more is the fracture susceptibility as well. Ideally d < 1/3 cross- sectional d of root
- Surface: Rough, serrated, abraded, notched, grooved or threaded posts provide more retention than smooth. According to Nergiz, roughening the canal surface via notching or grooving will enhance the retention of the prefabricated posts. Luting cements: Studies have shown that resin cements are the most retentive followed by Zn PO4, Type II GIC, RMGIC and finally polycarboxylate cements -
Number of posts: In multiple rooted teeth, more than 1 post can be used to increase retention and retain core especially in severely broken down teeth.
Some considerations for posterior teeth: -
Relatively long / circular posts are to be avoided in curved ribbon shaped canals. Short posts in divergent canals provide better retention.
A cast core can be made in sections (split casting) with different paths of with drawl
The widest canal is selected for the major post and short auxiliary post spaces are prepared in the other canals with same path of withdrawl
RESISTANCE FORM: It is defined as the ability of the post tooth to withstand lateral and rotational forces.
Resistance to stresses: is provided by -
1. Ferrule effect 2. Minimum internal preparation 3. Conservation of dentin apicaly and cervically as these are high stress zones 4. Increase in post length 5. Avoiding sharp angles 6. Preferring parallel over tapered posts as the former distributes stresses more evenly 7. Preferring vented parallel posts over smooth parallel posts (tapered posts are self -venting) 8. Stresses induced by threaded posts can be reduced by backing off ½ a turn 9. Cement layer results in more even stress distribution.
Rotational Resistance: is provided by -
If sufficient coronal tooth structure is present by the vertical / axial wall.
If insufficient then –
A groove / notch at the orifice of canal in the region of max bulk is placed. Then is called as anti-rotational notch/groove / element or keyway. An auxillary can be placed A cavity can be prepared in the post and adjacent tooth and amalgam can be condensed
In posterior teeth a post in another canal can be used to provide resistance to rotation.
CLINICAL TECHNIQUES: 1. Removal of endodontic filling material > Silver cone – Remove and reobturate with GP - Earlier sectional silver cone obturation was suggested done by prenotching 3mm above the cone and twisting off the coronal portion after placement. This should be clearly avoided as chances of dislodgement and apical leakage is very high > Gutta-percha The timing for removal of GP is still controversial. It was previously believed that 24 hours to a week delay was required to avoid disturbing the seal. Recent reports reveal that adequately condensed GP can be removed immediately. a) Non-carrier based obturation (lat/warm vertical/ injectable): GP can be removed via : -
Heat using pluggers / system B /Touch ‘n heat.
Chemical using solvents like chloroform, xylene, halothane turpentine etc.
Mechanical using rotary instruments like GG drills, Peeso reamers, Parapost drills NiTi rotary –instruments.
Of all these methods, fastest, safest and easiest method is the mechanical method. If immediately removal is needed, then heat is the preferred method to avoid distributing the seal. b) If a carrier based obturation like Thermafil was planned, then pre-notching of carrier 3-4mm above the apical extent can be done and the coronal portion twisted off after insertion. A radiograph is taken to confirm adequate removal of GP.
2. Post selection: as mentioned before depends on root morphology remaining coronal tooth structure, occlusal forces and esthetic concerns. 3. Enlargement of canal: Is done to remove undercuts and prepare the canal to receive an appropriately sized post. As mentioned before the enlargement should not be more than 1/3rd the diameter of the root with at least 1mm of sound dentin surrounding the canal. 4. Coronal tooth structure preparation -
Remove undermined enamel
Prepare as though tooth was undamaged
Prepare finish line at-least 2mm gingival to core to attain adequate ferrule design
A contra bevel can be placed at junction of future core and tooth to provide an additional metal collar from the core. This helps to:
ďƒ˜ Brace tooth against fracture ďƒ˜ Provides a vertical stop against overseating ďƒ˜ Reduces wedging effect - At times because of excessive flaring / then dentinal walls due to caries extension a post can be cemented prior to crown preparation to protect the weakened tooth structure Another approach is to fill the canal space with composite around a Luminex light transmitting post, which transfers light to the composite but does not bond to it. The post space thus created is treated routinely using peeso or low speed drills. -
If anti-rotational key way is required, a fracture 170 bur is used to make a groove /
notch extending 4mm into the canal to a depth of 0.6mm at the canal orifice at its bulkiest portion - Eliminate all sharp angles
5. Post Fabrication:
Custom –cast: a) Direct Procedures -
Select sprue formers [wood/ plastic], which slide easily into canal till apical end without binding.
After lubricating the canal, coat sprue former with inlay wax, place in canal and acquire a with inlay wax, place in canal and acquire a proper impression of the canal space.
Form a core with additional wax in the shape of the final tooth preparation
This procedure can be done using cold cure acrylic instead of wax.
b) Indirect Procedures: -
An orthodontic ‘J’ shaped wire is verified for loose fit full length in the canal and coated with an adhesive
Canal is lubricated.
Fill canal with elastomeric impression material
Using a lentulospiral -
Seat the wire and syringe in more impression material and seat the tray
Pour the cast and fabricate the post and core on the cast done in the direct method
Investing and Casting: Some important points to remember is: -
1-2 cc of extra water is added to the investment a liner is omitted. This is done to increase the casting shrinkage, which would result in a slightly smaller post that does not bind to the canal and provides space for the cement. A tight fit could cause root fracture.
A provisional restoration: Made of an orthodontic wire within the canal a temporary acrylic JC should be provided to the patient.
All temporary materials are removed
Canal is dried irrigation with EDTA and NaOCl
Fit of post and core is checked with light pressure
If it binds air abrade the post, reinsert, check for shiny spots and relieve them.
Finish polish the core
Place vertical groove if needed on the post to serve as an escape vent for the cement
Coat as canal with cement using a lentulo spiral
Coat post with cement and slowly place the post and core into place.
For a prefabricated post, an appropriate post with adequate length and diameter is selected. The length is adjusted so as to leave 2-3mm of it supragingival for core retention. The core is built up over this.
If cementation is done resin cement is planned then the method is: -
Mix and place primer in dried canal
Mix resin cement and coat the post with it
Do not place paste in canal as it will set almost instantly an contacting primer
Place post in canal and apply pressure for one-minute light cure it.
Apply oxyguard for 3 minutes and wash it off
An advancement in the fiber reinforced post system is the poly ethylene woven ribbon fibers or the Ribbond fiber post. Developed by Frielich, it involves the packing of several strips of resin saturated Ribbond fibers into a conditional and primed post space along with dual-cure resin cement. A core of composite is fabricated over this.
The all ceramic post and core are fabricated by: Slip casting
Copy milling Two piece technique = Zr post + copy milled ceramic core Heat pressed technique = Zr post + pressed glass ceramic core Even though endodontically treated teeth with their restorations have excellent prognosis failures do occur a failure rate of 5-17% has been sited for post and core restry. These are mainly due to: 1. Root fracture 2. Post fracture 3. Secondary caries 4. Periodontal disease Careful case selection, adherence to Biomechanical principles, appropriate post selection and meticulous maintenance of oral hygiene can prevent these problems.
CONCLUSION: “ SO NEAR…BUT YET SO FAR” From a simple piece of wire in 352AD, to the wooden post of the 18 th century to the esthetic post of today, history has borne witness to the relentless efforts made to secure an artificial tooth to the root. Judging by the pace at which research is moving, combining material science and understanding with dental relevance, that day is not far when one can provide an accurate and complete dental rehabilitation to an endodontically treated tooth in minimal chair-side time with maximum satisfactory results. Here’s looking forward to a 100% success rate for endodontics.